1. Field of the Invention
This invention relates generally to the preparation of ferrites, including garnets, and more particularly to the preparation of ferrites and garnets from a mixture of metal salts of dicarboxylic acids, for use in microwave elements such as isolators, circulators, phase shifters and transmission line components.
2. Description of the Prior Art
Polycrystalline ferrite components for microwave applications are usually made by traditional ceramic processes using ceramic powders which are formed by prereaction of constituent oxides or carbonates at high temperature, that is, greater than 1000.degree. C.
As is well known to those skilled in the art, and as used herein the term "ferrite" relates to magnetic oxides containing iron oxide as their main component and includes both the spinel and garnet crystal structures.
One of the most versatile of these ferrites is magnesium ferrite, which possesses the structure of the mineral spinel, MgAl.sub.2 O.sub.4, where iron (Fe) replaces the aluminum (Al) atom in the crystal lattice. The versatility of basic magnesium ferrite for microwave applications is demonstrated by the range of magnetic properties available by substitution of divalent ions other than Mg in the spinel structure.
It is well known in the industry that devices operating at low microwave frequencies require materials with low saturation magnetization (4 .pi.M.sub.s), and materials with high operating frequencies require materials with high 4 .pi.M.sub.s. To meet this industrial demand for radar and microwave ferrites with a wide range of saturation magnetizations, as well as to tailor other properties such as dielectric loss tangent and ferromagnetic linewidth, the substitution of various elements has been implemented.
Those skilled in the art of producing Mg and Ni ferrites for example, substitute Zn for the Mg or Ni, represented by Me, to about x=0.5 in the general formula Me.sub.1-x Zn.sub.x Fe.sub.2 O.sub.4. This has the effect of increasing the 4 .pi.Ms. Likewise, Mn is known to reduce dielectric losses in microwave ferrites when similarly substituted in fractions of 0.005&lt;.times.&lt;0.3, but more commonly in the 0.05&lt;.times.&lt;0.15 range. It may be added in combination with zinc or other Me. Note that the above equation is merely exemplary. In practice the MeFe.sub.2 O.sub.4 may have excess Me, (where Me is a 2+ metal ion), that is, the Me:Fe ratio is greater than 1:2.
Lithium ferrites with Me substitutions have the general formula Li.sub.1+x2 Me.sub.x Fe.sub.2.5-x/2 O.sub.4. Those skilled in the art of producing Li ferrites for example, substitute Zn, Ni, and Me either singly or in combination to tailor the desired magnetic and dielectric properties. There may be some deviation from this general formula for this ferrite as well.
The transition elements Mn, Fe, Co, Ni, Cu and Zn, and also Li, Ba, and Cd are examples of elements which, when substituted singly or in combination in ferrite compositions, form various ferrites, for example MnFe.sub.2 O.sub.4, NiFe.sub.2 O.sub.4, and Li.sub.0.5 Fe.sub.2.5 O.sub.4. When they are combined, these materials are referred to as mixed ferrites, mixed crystals or solid solution ferrites. Aluminum and titanium are used to reduce the magnetization. In lithium ferrite, the addition of Co can change the sign of the anisotropy constant of the material and also provides a range of spin wave line widths. The inclusion of nickel improves magnetic hysteresis loop squareness.
The cation distribution in the spinel lattice in practical combinations of mixed ferrites defines the properties, and can be affected by firing conditions (temperature, atmosphere, and cooling rate) and chemical purity. Lithium ferrites are susceptible to lithium loss at elevated temperatures (&gt;1000.degree. C.) and magnesium and lithium ferrites exhibit oxygen non-stoichiometry at high temperatures. Magnesium and lithium ferrites can be fired in air or oxygen. Manganese ferrites usually require control of oxygen partial pressure during firing. The sintering temperature for magnesium ferrite is normally high, greater than 1250.degree. C. especially when the starting powder is made by conventional means. The sintering temperature of lithium ferrite is typically &lt;1100.degree. C. Sintering aids such as bismuth oxide or vanadium oxide may be useful for lithium ferrite, but not magnesium ferrite. Copper oxide or glass forming additives such as silicates or borates may assist densification of Mg ferrite, but the microwave dielectric properties are degraded.
Related magnetic ceramic ferrites with the garnet structure are also useful in a variety of applications due to the wide range of elemental substitutions which enables versatility in magnetic and microwave properties. Magnetic garnets with the general formula Y.sub.3 Fe.sub.5 O.sub.12 often include multiple substitutions, such as gadolinium, holmium, aluminum and others. The general formula for a garnet with multiple substitutions is Y.sub.3-x Gd.sub.x Fe.sub.5-y Al.sub.y O.sub.12. The present invention is useful for these compositions as well.
Doping with small amounts of relaxing ions can drastically change the anisotropies and ferromagnetic resonance. All cations with ionic radii between 0.26 and 1.29 angstroms can be incorporated to tailor the garnet's properties. Examples of useful garnet devices are phase shifters and limiters.
Conventional ceramic powders are made by mixing the oxides or carbonates, calcining, and then milling the reactant. This may be repeated a number of times to achieve chemical homogeneity. For magnesium ferrite, for example, iron oxide and magnesium carbonate powders in the proper ratios are ball milled to mix and pulverize the powders and are then calcined within the range of 800.degree. C. to 1450.degree. C. Note that lithium ferrites are usually calcined at &lt;1000.degree. C., since lithium loss will occur otherwise. The reaction product is again milled and is then spray-dried with a dry pressing binder. The spray dried powder is pressed into a bar or similar shape and fired at 1200-1400.degree. C. whereupon it becomes hard and dense. Final characterization, cutting and machining follow.
The machined ceramic component is then metallized with gold by sputtering, screen printing, or similar methods, which produces the desired microwave circuitry.
Ferrites made by conventional methods are subject to variability in each of the many processing steps, and without very careful process control, their dielectric and magnetic properties vary, and their dielectric losses and ferromagnetic resonance linewidths are higher than desired. The effect of this variability is that the microwave system in which the material is employed does not perform optimally. This is especially true in systems using large antenna arrays, where matching antenna and transmission line elements to close tolerances is required.
For some microwave system components, small and uniform grain size is critical for optimum functioning of the ceramic, however, heat treatment of the powders causes grain growth. The fine and uniform sub-micron-sized particles desired can never be achieved with these powders, even with extensive milling. Therefore, conventional processing yields grains that are already large, limiting the ultimate performance of the ceramic device and system.
Moreover, the magnesium ferrite powders of the prior art are fired at high temperatures in excess of 1200.degree. C. to achieve high density, and this prevents co-firing with metals such as gold, as well as co-firing with most other ceramics used in microwave packaging because of chemical interaction at these high temperatures.
Co-precipitation of ferrites has been known since the mid-1950s via the oxidation of mixed metal hydroxides (G. Economos, J. Am. Ceram Soc., 38,241, 1955). Furthermore, coprecipitation of a mixture of oxalates is a well-known step in preparation of ferrites. Wickam (Inorg. Synth., 9,152 (1967)) and Paris (Thesis, University of Lyon, 1963) describe examples of preparing magnesium ferrites from oxalates. In this process, metal acetates are dissolved in a solution of aqueous acetic acid, which is deaerated by bubbling nitrogen. Iron powder is then added. When vigorous evolution of hydrogen has stopped, the mixture is heated to reflux in order to dissolve the iron completely. The final step of the precipitation is addition of an aqueous oxalic acid solution at elevated temperatures causing formation and precipitation of the mixed oxalates. Rigorous exclusion of oxygen is important in this step because Fe(II) is readily converted to Fe(III) by oxygen under these conditions. The Fe(III) oxalate is soluble in this solution. Thus if any is present, the stoichiometry of the mixed oxalate precipitate would be low in iron content. It is commonly believed that because the oxalate ion is bidentate, the product is a coordination polymer containing all the metal ions present in solution. This method works well for compositions in which all of the metal acetates are soluble in acetic acid and all of the metal oxalates are insoluble under these conditions.
There are, however, several shortcomings of this approach. Primarily, the process must be carried out in the absence of air to prevent Fe(III) formation. Moreover, the dissolution of iron must be carried out slowly because of the vigorous evolution of hydrogen. The coprecipitation of a mixture of oxalates will not work for compositions in which the acetates (or other simple salts) are not soluble, such as those containing aluminum, or for compositions in which the oxalates are soluble, such as compositions containing Li or Ti. In addition, some elements such as titanium or aluminum may be desired in the composition, but may be difficult to precipitate due to formation of complexes or highly pH sensitive solubility. Coprecipitation of a mixture of oxalates will not work for compositions including a metal having a reduction potential greater than Fe .sup.2+ (-0.447 V vs. H.sub.2 /H.sup.+). Where the reduction potential is greater, such as for Ni or Cu, unwanted metal flakes are formed by reduction of the metal salts in the acetate solution. Finally, there is incomplete control over the metal content of the product because of slightly residual solubility of metal oxalates.
Because of the solubility of lithium hydroxide and oxalate, the hydroxide and oxalate coprecipitation methods are not practicable for lithium ferrites. However, lithium ferrite is also a very versatile ferrite; its magnetic properties can be altered to cover a wide range of values. It differs from magnesium ferrite because it can easily be fired to high density at less than 1000.degree. C. Compositions containing metals such as zinc substituted for part of the iron oxide, and bismuth oxide as a sintering aid are shown to densify at low temperature by D. W. Johnson et. al., ("Effect of Preparation Technique and Calcination Temperature on the Densification of Lithium Ferrites", Bull. Am. Cer. Soc., v.53,2, 1974.) These authors evaluated powders made by conventional(oxide) or non-conventional means, such as precipitation, spray pyrolysis, and freeze drying.
A. Micheli coprecipitated lithium stearate and metal hydroxides("Preparation of Lithium Ferrites by Co-precipitation," IEEE Trans. on Magnetics, p.606-608, September 1970). Pechev and Pecheva spray dried lithium-iron formate solutions and calcined the product to form uniform ferrite powder ("A Study of Sintering and Magnetic Parameters of Spinel Lithium Ferrite", Mat. Res. Bull., Vol. 15, pp 1199-1205, 1980). Oda, et. al. ("Preparation of LiFe.sub.5 O.sub.8 by the Sol-Gel Method", J. Mat. Sci. Let., 5, 1986 545-548) derived ferrite from alkoxides of lithium and iron. C. Barriga, et. al., describe a variant of the precipitation method ("Lithium Ferrite Formation by Precipitation form Fe(III) Solutions, J. Sol. State Chem, 77, 132-140, (1988)).
Powders made by these methods can be used to fabricate microwave elements such as isolators, phase shifters, circulators and limiters. These circuit elements can be used in very low power receiver circuits to multi-kilowatt phased array high power applications. Examples of various shapes include thin films and laminated tapes or toroids and rod-shaped billets.
The manufacture and use of such microwave elements is discussed in greater detail in copending application Ser. No. 08/685,885, entitled Process of Fabricating a Microwave Power Device, filed Jul. 25, 1996.
Because of the importance and variety of uses for microwave elements, there exists a need for highly reliable and low cost methods of ceramic powder production that result in very pure and uniform material. Therefore, the present invention describes novel methods of powder production that provide nearly the chemical homogeneity achievable with coprecipitation, while maintaining the purity, surface area and particle size needed for uniform sintering and densification. Finer grain sizes which effect the properties of the resulting ceramic can also be achieved with these methods.